In adaptive radiation therapy, treatments are changed (adapted) on a periodic basis to account for changes in the patient's anatomy. For the detection of anatomical changes, an image is periodically acquired using a tomographic technique. The new (registered) images are compared to the initial (reference) image that was used for the patient's initial treatment plan. Deviations from the reference image caused by, e.g., tumor shrinkage or organ movement, are then used to adapt the course of treatment undertaken by the attending physicians.
In order to compare two such images, correspondences are normally established by calculation of a deformation map. The deformation map contains, for each pixel in the registered image, a three-dimensional vector that describes how this pixel has moved between the reference image and the registered image. Thus, the deformation map contains complete information about changes in geometry in the section of interest within the patient's anatomy. The deformation map typically comprises a vector field. However, the internal storage implementation of the deformation map in the associated computer can differ from a vector field representation, in which case said implementation can be converted to a vector field automatically or upon demand of the user.
In existing systems, volumetric (three-dimensional) patient images are usually visualized by displaying all pixels at the intersections of the 3D volume with arbitrarily-oriented user-definable two-dimensional slice planes. Pairs of registered images are then displayed, by first deforming the reference image into the space of the registered image using the deformation map, and then by displaying these slices either as side-by-side comparisons or as blended overlay images. While such visualization techniques can serve to compare both images, they do not provide any information about particular local properties of the deformations. The user can see only whether or not the images match, but cannot observe the degree of deformation at particular places.
These issues are addressed by the present invention, which provides novel techniques for visualization of deformations, e.g., by using color overlays.